A mixture of the type stated at the outset has already been described in EP-A 0 369 645. It comprises a monomer capable of free radical polymerization and a photoinitiator system which is soluble therein and contains a trihalomethyl-substituted 1,3,5-triazine, a sensitizer for the triazine and an electron donor compound having an oxidation potential greater than zero and less than that of 1,4-dimethoxybenzene. The sensitizers mentioned are coumarin, xanthene, acridine, thiazole, thiazine, oxazine, azine, aminoketone, methine and polymethine dyes, porphyrins, aminotriarylmethanes, merocyanines, squarylium and pyridinium dyes. They are sensitive in particular to radiation in the range from 350 to 700 nm. For radiation in the near IR range (700 to 1 200 nm), on the other hand, the sensitivity is only low.
Furthermore, EP-A 0 315 988 discloses a photopolymerizable mixture which is sensitive to radiation in the range from 600 to 700 nm. It comprises a polymerizable, ethylenically unsaturated compound, a free radical former and a substituted 2-phenyl-2H-naphtho[6,5,4-a,m,n]thioxanthene-1,3-dione or a substituted 3-alkoxy-2-phenyl-naphtho[6,5,4-a,m,n]thioxanthen-1-one as a sensitizer.
EP-A 0 441 542 relates to a composition crosslinking on irradiation and comprising a polymer to which mono-, di- or trihalomethyl-substituted [1,3,5]triazine radicals are covalently bonded via a bridging group. The base polymer can be selected from a large number of polymers. It may be, for example, a polyamide, a polyester, a polyurethane, polysiloxane, a phenol resin, a polystyrene, a polyacrylate, a polyacrylic acid, a polyacrylamide, a polyacrylonitrile, a polyethylene, a polybutadiene, polyvinylpyrrolidone, polycaprolactone, gelatin, starch or a polysaccharide.
EP-A 0 563 925 discloses a photopolymerizable mixture which comprises a monomer capable of free radical polymerization, a substituted 2-phenyl-4-halomethyl- (or 4,6-bishalomethyl)-[1,3,5]triazine and a compound which acts as a sensitizer for the triazine. The mixture is used for the production of negative-working printing plates.
A directly imagable recording material for the production of planographic printing plates is described in EP-A 1 106 381. It comprises an electrochemically grained substrate of a special aluminum alloy and a photosensitive layer which contains an IR absorber and a water-insoluble but alkali-soluble polymeric binder.
The recording material directly imagable by means of UV, VIS or IR laser radiation and intended for the production of offset printing plates according to EP-A 1 091 247 comprises a photosensitive layer which contains a monomer having one or more polymerizable, substituted acrylate group(s) or derivatives thereof and a photopolymerization initiator. In the α-position relative to the polymerizable double bond of the monomer, there is always a hetero atom or a halogen atom. Preferred initiators are aromatic ketones, aromatic onium salts, organic peroxides, hexaarylbisimidazoles, borates, metallocenes and compounds having carbon-halogen bonds. The last-mentioned may also include substituted [1,3,5]triazines having trihalomethyl groups. The photosensitive layer may also contain sensitizer dyes. A polyvinyl alcohol layer may additionally be present on the photosensitive layer. An important disadvantage of this recording material is that the monomers are difficult to synthesize and hence correspondingly expensive. Furthermore, the reactivity and hence the photosensitivity of the layer are reduced.